1. Field of the Invention
This invention relates to helicopter rotor blades and is particularly concerned with rotor blades of the construction disclosed in GB-A-1538055.
2. Description of the Prior Art
Rotor blades according to GB-A-1538055 are characterised by a swept tip portion which has a forwardly swept leading edge portion, a rearwardly swept leading edge, a rearwardly swept extreme tip edge, and a rearwardly swept trailing edge. The region of the forwardly swept leading edge portion has become known as the notch region and in practice the blended arrangement disclosed in FIG. 3 of GB-A-1538055 has been preferred, the blend comprising blend radii and a central straight joining portion at an angle of about 60 degrees from a line parallel to a blade feathering axis. An inner blend curve blends directly with the blade leading edge and an outer blend curve blends with the rearwardly swept leading edge. Apart from a small residual twist to cater for a change of aerofoil section throughout the notch region on an otherwise nominally about 8 degree linearly twisted blade there is no additional twist in the notch region to tune the aerodynamic characteristics of the notch region.
Such rotor blades have proved highly successful in producing large increases in the allowable rotor operating envelope, and were a significant factor in the establishment of the world absolute speed record for helicopters of 249.10 mph (400.81 km/hr) set by a Westland Lynx helicopter in 1986.
In continuing efforts to improve helicopter performance, wind tunnel tests of the prior rotor blades revealed a tendency for an area of separated flow to occur on an upper surface of the blade behind the notch region. The problem occurs at high angles of incidence in a range typically encountered by the retreating blade of a helicopter sustaining rotor and the size of the separated area increases as incidence is increased.
An objective of this invention is therefore to reduce or eliminate the area of separated flow behind the notch region of a helicopter rotor blade of the construction disclosed in GB-A-1538055.